Interpretive Summary: Feeder calves that are shipped to feedyards often encounter a number of stressors that may include periods without feed and water, adverse weather conditions, etc. Upon arrival at the feedyard these calves routinely have low feed intakes for one to three weeks. It is not clear what physiological mechanisms control feed intake in these stressed calves, although hormonal changes and depressed microbial activity in the rumen seem to be involved. When ruminants eat, water from the plasma and extracellular compartments moves to the rumen to buffer osmotic changes that occur. In a partially dehydrated animal, such a shift in body water could have adverse effects. Thus, feed intake may be limited in order to prevent these adverse effects. This research was conducted to determine the effects of partial dehydration, induced by a 72 hour period without feed and water, on shifts in body water during a meal using lambs as a model for feeder calves. After feeding, extracelluar water space decrease and ruminal volume increased in both fed and deprived wethers although the changes were greater in fed sheep. Changes in plasma and ruminal fluid concentrations and quantities of the electrolytes Na, K, and Mg also suggested that electrolyte shifts after a meal differed in fed and deprived wethers. Although additional studies are needed, these results suggest that altered body water and electrolyte shifts may be a regulating factor in the decreased feed intake of feeder calves stressed by marketing and transport.

Technical Abstract:
Post-prandial shifts in body water compartments might limit feed intake by calves and sheep stressed as a result of marketing and transport. This experiment was conducted to determine the effects of feed and water deprivation on post-prandial changes in body water compartments in wethers. Eight Hampshire wethers (average BW 42 +/- 2 kg) were used in a crossover design. During each period, half the wethers were limit-fed (540 g DM/d: FED) and half were deprived of feed and water for 72 h (DEPRIVED). Wethers were infused i.v. with Evans blue and sodium thiosulfate and intraruminally with Cr- or Co-EDTA after which blood and ruminal samples were collected for the next 4 h. All wethers were then fed 540 g of feed DM, and infusions were repeated 30 min after feeding. Body water compartment volumes were determined by linear regression of plasma concentrations of Evans blue (plasma volume), and sodium thiosulfate (extracellular volume), and by ruminal fluid concentrations of Cr or Co. Feed and water deprivatio decreased (P < .01) extracellular water space but did not affect plasma or ruminal water space. After feeding, extracelluar water space decreased P <.01) and ruminal volume increased (P < .05) in both FED and DEPRIVED wethers. Plasma pools of Na, K, and Mg were not affected by feeding in FED wethers but decreased (P < .05) in DEPRIVED wethers. The increase in ruminal fluid pools of Na, K and Mg were appreciably greater (P < .05) in FED than DEPRIVED wethers. Additional studies are needed to determine whether these water and electrolyte shifts, altered ruminal fermentation, and metabolic changes are involved in regulating the decreased feed intake by ruminants subjected to marketing and transport stress.